In a rechargeable battery, as in other conventional electrochemical systems during current flow, the voltage across the terminals is influenced by the electrochemical reactions which occur near the battery electrodes, by polarization near the electrodes and by the internal resistance of each half cell.
Methods are known for measuring the quality of a battery in which the over-all internal resistance of the galvanic cell or battery is measured by a pulsed current. Examples of such techniques are described in U.S. Pat. Nos. 2,662,211 and 2,864,055 and in an article by K. Koresch and A. Marko entitled "Sine Wave Pulse Current Tester for Batteries" J.E.C.S. Vol. 107, p. 480 (1960). In these methods it is assumed that the voltage drop attributable to polarization remains substantially constant when the load or the current source is disconnected for a short period. A pulsed current, rather than a constant current is therefore passed through the cell and the voltage of the cell is measured between pulses when the current is zero. The measured voltage is termed the "modified terminal voltage" and differs from the "terminal voltage" usually measured across the load by the current-resistance potential drop.
The modified terminal voltage is used to determine the battery capacity. In conventional practice, the battery is charged at a rapid rate with a constant current for a short period and the modified terminal voltage is measured at the beginning and end of a predetermined interval within that period. The differences in measured modified terminal voltage indicate changes in battery capacity. A similar measurement can be made during battery discharge.
By comparing a plurality of batteries of the same type during periodic charging and discharging, it is possible to indicate which of the batteries are exhausted. Such conventional techniques are not, however, helpful when it is sought to determine the capacity of newly designed or nonstandard batteries whose characteristics are not known in advance, and where each half cell of the battery must be checked independently.